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Abstract:

The present invention relates to an image sensor comprising a microlens
array, and to a manufacturing method thereof. The method of the present
invention includes gradually increasing the aluminum composition ratio of
a compound semiconductor as the latter gradually gets farther from a
substrate, to enable a microlens-forming layer to grow, and making the
oxidation rate of the region adjacent to the substrate slower and the
oxidation rate of the region farther from the substrate faster, making
the interface between the oxidized region and the unoxidized region into
a lens shape after the completion of oxidation. The thus-made lens is
integrated into an image sensor. The present invention reduces costs for
manufacturing image sensors in which a microlens is integrated, increases
the signal-to-noise ratio and resolution of the image sensor, and
achieves improved sensitivity.

Claims:

1. An image sensor including a microlens array, comprising: a substrate
having one side on which a plurality of photo-detectors configured to
sense light are formed; and a plurality of microlenses disposed on the
other side of the substrate and spaced a predetermined distance apart
from one another, the plurality of microlenses respectively corresponding
to the plurality of photo-detectors and configured to focus external
light and allow the light to be incident to the photo-detectors, wherein
each of the microlenses includes a lens-shaped semiconductor material
layer stacked such that an oxidation rate of the semiconductor material
layer gradually increases as the semiconductor material layer becomes
farther from the substrate, and wherein each of the microlenses includes
a plurality of different layers, the each of the plurality of layers
includes a digital alloy formed by stacking at least two semiconductor
material layers having different oxidation rates.

2. The image sensor of claim 1, wherein each of the microlenses is formed
by selectively oxidizing the semiconductor material layer.

3. The image sensor of claim 1, wherein the oxidation rate of each of the
plurality of layers is gradually increased by controlling the thickness
of a layer having a highest oxidation rate out of the at least two
semiconductor material layers having the different oxidation rates.

4. The image sensor of claim 3, wherein the semiconductor material layer
includes a combination of an aluminum (Al)-containing ternary or
quaternary compound and is formed by stacking an Al-containing binary or
ternary compound and an Al-free binary or ternary compound.

6. The image sensor of claim 1, wherein a horizontal section of each of
the microlenses has a circular or polygonal shape.

7. The image sensor of claim 1, wherein each of the microlenses is a
spheric or aspheric lens.

8. The image sensor of claim 1, wherein a central portion of each of the
microlenses has a height of about 1 to about 2 μm.

9. A method of manufacturing an image sensor including a microlens array,
the method comprising: (a) forming a microlens-forming layer on one side
of a substrate by stacking a semiconductor material layer whose oxidation
rate is gradually increased as the semiconductor material layer becomes
farther from the substrate; (b) forming a plurality of mesa structures by
etching a predetermined region of the microlens-forming layer until the
substrate is exposed, the plurality of mesa structures spaced a
predetermined distance apart from one another and having exposed lateral
surfaces, respectively; (c) oxidizing a lateral surface of each of the
mesa structures while increasing an oxidation rate as each of the mesa
structures becomes farther from the substrate to make an interface
between an oxidized region and an unoxidized region into a lens shape
after completion of oxidation to form microlenses having a radius of
curvature in the centers of the respective mesa structures, and
selectively removing oxidized regions other than the microlenses; and (d)
forming a plurality of photo-detectors on the other side of the substrate
to respectively correspond to the microlenses.

10. The method of claim 9, further comprising, after step (a), forming an
oxidation barrier layer on a top surface of the microlens-forming layer
to a predetermined thickness.

11. The method of claim 9, wherein, in step (a), the microlens-forming
layer is formed using a plurality of different layers, wherein each of
the plurality of layers includes a digital alloy formed by stacking at
least two semiconductor material layers having different oxidation rates,
and an oxidation rate of each of the plurality of layers is gradually
increased by controlling the thickness of a layer having a highest
oxidation rate out of the at least two semiconductor material layers
having the different oxidation rates.

12. The method of claim 9, wherein, in step (a), the microlens-forming
layer includes a combination of an Al-containing ternary or quaternary
compound and is formed by alternately stacking an Al-containing binary or
ternary compound and an Al-free binary or ternary compound.

13. The method of claim 9, wherein, in step (b), each of the mesa
structures is formed as a circular or polygonal mesa structure.

14. The method of claim 9, wherein, in step (c), each of the microlenses
is formed in a lens shape having a radius of curvature by exponentially
oxidizing the microlens-forming layer according to the oxidation rate of
the semiconductor material layer stacked by gradually increasing the
oxidation rate in the microlens-forming layer.

15. The method of claim 9, wherein, in step (c), the oxidizing of the
lateral surface of each of the mesa structures is performed using a wet
oxidation process at a temperature of about 300 to about 500.degree. C.
for about 30 to about 200 minutes.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of U.S. application
Ser. No. 13/133,940, filed on Aug. 9, 2011, which is a National Phase of
International Application No. PCT/KR2009/006442, filed on Nov. 4, 2009,
and which claims priority to and the benefit of Korean Patent Application
No. 10-2008-0126145, filed on Dec. 11, 2008, and the disclosures of which
are hereby incorporated herein by reference in their entireties.

BACKGROUND

[0002] 1. Field of the Invention

[0003] The present invention relates to an image sensor including a
microlens array and a method of manufacturing the same, and more
particularly, to an image sensor including a microlens array and a method
of manufacturing the same, by which a lens may be manufactured using a
semiconductor material. The method enables a lens to be monolithic or
hybrid integrated with an image sensor, which makes the image sensor have
a high numerical aperture (NA), a subminiature, high-density, polygonal
microlens array.

[0004] 2. Discussion of Related Art

[0005] Generally, the improvement of signal-to-noise ratios (SNR),
resolution, and sensitivity has become an important issue in developments
of image sensors, particularly, high-performance infrared (IR) image
sensors.

[0006] A main cause of a noise of an IR image sensor may be a dark
current. Since the dark current is linearly proportional to the area of
the photodetector, a photodetection region should be reduced to improve
an SNR.

[0007] In addition, since the number of pixels per unit area should be
increased to improve the resolution of an image sensor, the area of each
pixel including a photodetection region should be reduced.

[0008] However, a reduction in the photodetection region may lead to a
reduction in the amount of light received by a photodetector, which may
bring about a reduction in the light reception efficiency of the
photodetector, thus resulting in a drop in the sensitivity of the image
sensor.

[0009] Thus, a microlens array including a plurality of microlenses may be
formed on an image sensor. The microlens array, which may focus light
incident to a pixel on a small photodetection region of the pixel, may
function to effectively increase the amount of light incident to a
photodetector even if the photodetection region is reduced.

[0010] Accordingly, many methods of manufacturing a microlens array
integrated in an image sensor have been proposed thus far. A
representative method of manufacturing an image sensor including a
microlens array may include reflowing a polymer.

[0011] A method of reflowing polymer may include forming a transparent
predeposition layer on a substrate including an image sensor, forming
photoresist patterns for a plurality of microlenses on the transparent
predeposition layer, and reflowing the photoresist patterns by heating.
As a result, since the polymer tends to have a globular surface due to
surface tension, a microlens array having a predetermined radius of
curvature may be formed.

[0012] However, each of the microlenses formed using the above-described
reflow method may have a naturally circular shape, and thus the fill
factor of the microlens array (i.e., a ratio of the area of the microlens
to that of a square unit pixel region or unit cell region of the
microlens array) may be limited to 78% or lower.

[0013] That is, at least 22% of light incident to each of square pixels
may be incident to a region where no microlens is formed. Accordingly, at
least 22% of the light incident to each of the pixels may not be focused
on the photodetection region but be lost without contributing to forming
images.

[0014] Furthermore, since a method of manufacturing a microlens array
using the reflow of a polymer was performed using surface tension, it was
difficult to control the radius of curvature or focal distance of the
microlens, which doesn't allow a high-density of microlens array.

SUMMARY OF THE INVENTION

[0015] The present invention is directed to an image sensor including a
microlens array and a method of manufacturing the same, which facilitate
controlling the radius of curvature or focal distance of microlenses and
enable the manufacture of not only circular microlenses but also
polygonal microlenses.

[0016] The present invention is also directed to an image sensor including
a microlens array and a method of manufacturing the same, which may
reduce costs for manufacturing image sensors, increase the
signal-to-noise ratio (SNR) and resolution of the image sensors, and
improve the sensitivity thereof.

[0017] According to an aspect of the present invention, there is provided
an image sensor including a microlens array, including: a substrate
having one side on which a plurality of photo-detectors configured to
sense light are formed; and a plurality of microlenses disposed on the
other side of the substrate and spaced a predetermined distance apart
from one another, the plurality of microlenses respectively corresponding
to the plurality of photo-detectors and configured to focus external
light and allow the light to be incident to the photo-detectors. Each of
the microlenses includes a lens-shaped semiconductor material layer
stacked such that an oxidation rate of the semiconductor material layer
gradually increases as the semiconductor material layer becomes farther
from the substrate.

[0018] Each of the microlenses may be formed by selectively oxidizing the
semiconductor material layer.

[0019] Each of the microlenses may include a plurality of different
layers, each of the plurality of layers may include a digital alloy
formed by stacking at least two semiconductor material layers having
different oxidation rates, and the thickness of a layer having a highest
oxidation rate out of the at least two semiconductor material layers may
increase as the semiconductor material layer becomes farther from the
substrate.

[0020] The semiconductor material layer may include a combination of an
aluminum (Al)-containing ternary or quaternary compound and is formed by
stacking an Al-containing binary or ternary compound and an Al-free
binary or ternary compound.

[0022] A horizontal section of each of the microlenses may have a circular
or polygonal shape.

[0023] Each of the microlenses may be a spheric or aspheric lens.

[0024] A central portion of each of the microlenses may have a height of
about 1 to about 2 μm.

[0025] According to another aspect of the present invention, there is
provided an image sensor including a microlens array, including: a
microbolometer disposed on a first substrate and including a plurality of
thermal detectors configured to sense heat generated by infrared (IR)
light; and a microlens array including a plurality of microlenses formed
on one side of a second substrate, the plurality of microlenses
configured to focus external light incident from the other side of the
second substrate and allow the external light to be incident to each of
the thermal detectors. The microbolometer and the microlens array are
hybrid-integrated with each other such that the microlenses are
respectively spaced a predetermined distance apart from and opposite to
the corresponding thermal detectors.

[0026] Each of the microlenses may include a lens-shaped semiconductor
material layer stacked such that an oxidation rate of the semiconductor
material layer gradually increases as the semiconductor material layer
becomes farther from the second substrate.

[0027] According to another aspect of the present invention, there is
provided a method of manufacturing an image sensor including a microlens
array, the method including: (a) forming a microlens-forming layer on one
side of a substrate by stacking a semiconductor material layer whose
oxidation rate is gradually increased as the semiconductor material layer
becomes farther from the substrate; (b) forming a plurality of mesa
structures by etching a predetermined region of the microlens-forming
layer until the substrate is exposed, the plurality of mesa structures
spaced a predetermined distance apart from one another and having exposed
lateral surfaces, respectively; (c) oxidizing a lateral surface of each
of the mesa structures while increasing an oxidation rate as each of the
mesa structures becomes farther from the substrate to make an interface
between an oxidized region and an unoxidized region into a lens shape
after completion of oxidation to form microlenses having a radius of
curvature in the centers of the respective mesa structures and
selectively removing other oxidized regions than the microlenses; and (d)
forming a plurality of photo-detectors on the other side of the substrate
to respectively correspond to the microlenses.

[0028] After step (a), the method may further include forming an oxidation
barrier layer on a top surface of the microlens-forming layer to a
predetermined thickness.

[0029] In step (a), the microlens-forming layer may be formed using a
plurality of different layers. Each of the plurality of layers may
include a digital alloy formed by stacking at least two semiconductor
material layers having different oxidation rates, and an oxidation rate
of each of the plurality of layers may be gradually increased by
controlling the thickness of a layer having a highest oxidation rate out
of the at least two semiconductor material layers having the different
oxidation rates.

[0030] In step (a), the microlens-forming layer may include a combination
of an Al-containing ternary or quaternary compound and be formed by
alternately stacking an Al-containing binary or ternary compound and an
Al-free binary or ternary compound.

[0031] In step (b), each of the mesa structures may be formed as a
circular or polygonal mesa structure.

[0032] In step (c), each of the microlenses may be formed in a lens shape
having a radius of curvature by exponentially oxidizing the
microlens-forming layer according to the oxidation rate of the
semiconductor material layer stacked by gradually increasing the
oxidation rate in the microlens-forming layer.

[0033] In step (c), the oxidation of the lateral surface of each of the
mesa structures may be performed using a wet oxidation process at a
temperature of about 300 to about 500° C. for about 30 to about
200 minutes.

[0034] According to another aspect of the present invention, there is
provided a method of manufacturing an image sensor including a microlens
array, the method including: (a') manufacturing a microbolometer on a
first substrate, the microbolometer including a plurality of thermal
detectors configured to sense heat generated by IR light; (b')
manufacturing a microlens array including a plurality of microlenses on
one side of a second substrate, the plurality of microlenses configured
to focus external light incident from the other side of the second
substrate and allow the external light to be incident to each of the
thermal detectors; and (c') hybrid-integrating the separately
manufactured microbolometer and microlens array such that the thermal
detectors are respectively disposed a predetermined distance apart from
and opposite to the corresponding microlenses.

[0035] Step (b') may include: (b'-1) forming a microlens-forming layer on
one side of the second substrate by stacking a semiconductor material
layer whose oxidation rate is gradually increased as the semiconductor
material layer becomes farther from the second substrate; (b'-2) etching
a predetermined region of the microlens-forming layer until the second
substrate is exposed to form a plurality of mesa structures spaced a
predetermined distance apart from one another and having exposed lateral
surfaces; and (b'-3) oxidizing a lateral surface of each of the mesa
structures while increasing an oxidation rate as each of the mesa
structures becomes farther from the second substrate to make an interface
between an oxidized region and an unoxidized region into a lens shape
after completion of oxidation to form microlenses having a radius of
curvature in the centers of the respective mesa structures and
selectively removing other oxidized regions than the microlenses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The above and other objects, features and advantages of the present
invention will become more apparent to those of ordinary skill in the art
by describing in detail exemplary embodiments thereof with reference to
the accompanying drawings, in which:

[0037] FIG. 1 is a cross-sectional view of a microlens according to an
exemplary embodiment of the present invention;

[0038] FIG. 2 is a graph showing an aluminum (Al) content and oxidation
rate, which illustrates a method of forming a microlens using an analog
alloy process;

[0039] FIGS. 3 and 4 are cross-sectional views illustrating a method of
forming a microlens using a digital alloy process;

[0040] FIG. 5 is a cross-sectional view of an image sensor including a
monolithically integrated microlens array according to an exemplary
embodiment of the present invention;

[0041] FIG. 6 is a cross-sectional view of a microbolometer infrared (IR)
image sensor including a hybrid integrated microlens array according to
an exemplary embodiment of the present invention;

[0042] FIGS. 7 through 14 are cross-sectional views illustrating a method
of manufacturing an image sensor including a monolithically integrated
microlens array according to an exemplary embodiment of the present
invention; and

[0043] FIGS. 15 and 16 are optical microscopic images of a microlens array
manufactured using a method according to an exemplary embodiment of the
present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0044] Various exemplary embodiments of the present invention will now be
described more fully with reference to the accompanying drawings in which
some exemplary embodiments are shown. This invention may, however, be
embodied in different forms and should not be construed as limited to the
embodiments set forth herein. Rather, these embodiments are provided so
that this disclosure is thorough and complete and fully conveys the scope
of the present invention to one skilled in the art.

[0045] FIG. 1 is a cross-sectional view of a microlens according to an
exemplary embodiment of the present invention, FIG. 2 is a graph showing
an aluminum (Al) content and oxidation rate, which illustrates a method
of forming a microlens using an analog alloy process, and FIGS. 3 and 4
are cross-sectional views illustrating a method of forming a microlens
using a digital alloy process.

[0046] Referring to FIG. 1, a microlens L according to an exemplary
embodiment of the present invention may be formed on a prepared substrate
100. For example, a predetermined microlens array may be formed using a
combination of a plurality of microlenses L.

[0047] Each of the microlenses L may include a semiconductor material
layer stacked by gradually increasing an oxidation rate as the
corresponding microlens L becomes farther from the substrate 100. Each of
the microlenses L may be formed by selectively oxidizing the
semiconductor material layer.

[0048] For example, the oxidation rate of the semiconductor material layer
may depend on the Al composition ratio thereof. The formation of the
microlens L may include gradually increasing the Al composition ratio of
the semiconductor material layer as the microlens L becomes farther from
the substrate 100 to increase the oxidation rate according to the Al
composition ratio as the microlens L becomes farther from the substrate
100. Thus, after completion of oxidation, an interface between an
oxidized region and an unoxidized region may form a lens shape.

[0049] In this case, a method of forming the semiconductor material layer
constituting the microlens L may be performed using, for example, an
analog alloy process or a digital alloy process, but the present
invention is not limited thereto.

[0050] Referring to FIGS. 1 and 2, initially, the analog alloy process may
include growing the semiconductor material layer for the microlens L
while gradually increasing, for example, the amount of Al, to gradually
increase the oxidation rate of the semiconductor material layer. When the
Al content is gradually increased as shown in FIG. 2, the oxidation rate
of the semiconductor material layer may be exponentially increased so
that the interface between the oxidized region and the unoxidized region
can naturally assume a lens shape during the oxidation of the
semiconductor material layer.

[0051] Referring to FIGS. 3 and 4, in the digital alloy process, the
microlens L may be formed of, for example, a multinary compound
semiconductor material. Unit compound semiconductor materials of the
multinary compound semiconductor material may be alternately stacked and
oxidized to form a lens shape.

[0052] A method of manufacturing the microlens L using the digital alloy
process according to an exemplary embodiment of the present invention
will now be described in further detail. Referring to FIG. 3, a
predetermined microlens-forming layer 200 for the microlens L may be
deposited on a prepared substrate 100.

[0053] The microlens-forming layer 200 may include, for example, a
plurality of different layers as shown in FIG. 3. Each of the plurality
of layers may include at least two semiconductor material layers having
different oxidation rates, for example, a high-oxidation-rate layer 210
having a high Al content and a low-oxidation-rate layer 220 having a low
Al content. Each of the plurality of layers may be formed by alternately
stacking the high-oxidation-rate layer 210 and the low-oxidation-rate
layer 220. Naturally, each of the plurality of layers may include three
different layers, but the present invention is not limited thereto.

[0054] For example, the semiconductor material layer may be formed of an
Al-containing ternary compound (e.g., aluminum gallium arsenide (AlGaAs))
or an Al-containing quaternary compound (e.g., indium gallium aluminum
arsenide (InGaAlAs) or indium gallium aluminum phosphide (InGaAlP)). When
the semiconductor material layer is a ternary semiconductor material
layer, for example, an AlGaAs semiconductor material layer, the AlGaAs
semiconductor material layer may be generally formed by alternately
stacking, for example, an Al-containing binary compound layer (i.e., an
AlAs compound layer) and an Al-free binary compound layer (i.e., a GaAs
compound layer) to a small thickness.

[0055] In addition, when the semiconductor material layer is a quaternary
semiconductor material layer, for example, an InGaAlAs semiconductor
material layer, the InGaAlAs semiconductor material layer may be formed
by stacking an Al-containing InAlAs compound layer and an Al-free InGaAs
compound layer. Also, when the semiconductor material layer is, for
example, an InGaAlP compound layer, the InGaAlP compound layer may be
formed by stacking an Al-containing InAlP compound layer and an Al-free
InGaP compound layer.

[0056] In this case, as the high-oxidation-rate layer 210 gets farther
from the substrate 100, the thickness of the high-oxidation-rate layer
210 of the semiconductor material layer may increase in order for the
average Al content of the semiconductor material layer of the
microlens-forming layer 200 to increase as the semiconductor material
layer of the microlens-forming layer 200 becomes farther from the
substrate 100.

[0057] That is, when the semiconductor material layer is a ternary
semiconductor material layer, for example, an AlGaAs semiconductor
material layer, the AlGaAs semiconductor material layer may be formed by
alternately stacking an AlAs compound layer and a GaAs compound layer as
described above. In this case, an Al0.9Ga0.1As layer may be
formed by stacking an AlAs layer and a GaAs layer in a thickness ratio
of, for example, 90 to 10, while an Al0.99Ga0.01As layer may be
formed by stacking an AlAs layer and a GaAs layer in a thickness ratio
of, for example, 99 to 1.

[0058] For instance, assuming that a composition ratio x of an
AlxGa1-xAs layer is intended to increase, for example, from 0.9
to 0.99 in a direction in which the semiconductor material layer is
grown, when the AlAs layer and the GaAs layer are alternately stacked,
the thickness of the AlAs layer may be gradually increased. Thus,
although the AlAs layer and the GaAs layer are minutely repeated, the
average composition ratio x of the AlxGa1-xAs layer may
increase from 0.9 to 0.99. Naturally, the composition ratio x may be
selectively controlled within a wide range, but the present invention is
not limited thereto.

[0059] Meanwhile, an oxidation barrier layer 250 may be further formed on
a top surface of the microlens-forming layer 200 as needed. The oxidation
barrier layer 250 may inhibit vertical oxidation of a top portion of the
microlens-forming layer 200 and allow only horizontal oxidation of a
lateral portion of the microlens-forming layer 200 during subsequent
oxidation of the microlens-forming layer 200, thus resulting in formation
of a lens shape.

[0060] Subsequently, when the microlens-forming layer 200 is oxidized in a
lateral direction, the average Al content of the microlens-forming layer
200 may increase toward a top portion thereof, so that the microlens L
can be formed as shown in FIG. 4.

[0061] In this case, since the low-oxidation-rate layer 220 stacked on the
microlens-forming layer 200 is formed to a much smaller thickness than
the high-oxidation-rate layer 210, almost all regions of the
low-oxidation-rate layer 220 may be simultaneously oxidized along with
the oxidation of the high-oxidation-rate layer 210 during the oxidation
of the microlens-forming layer 200. As a result, almost all the regions
of the microlens-forming layer 200 except a region where the microlens L
is formed may be oxidized irrespective of the high-oxidation-rate layer
210 and the low-oxidation-rate layer 220.

[0062] Here, the Al-containing high-oxidation-rate layer 210 may be formed
to a thickness of, for example, several tens of Å to several hundreds
of Å, while the Al-free low-oxidation-rate layer 220 may be formed to
a thickness of, for example, about 10 Å.

[0063] In addition, the microlens-forming layer 200 may be formed by
stacking the high-oxidation-rate layer 210 and the low-oxidation-rate
layer 220, for example, about 100 times, but the present invention is not
limited thereto.

[0064] Meanwhile, a plurality of microlenses L may be formed on the
substrate 100, thereby manufacturing a microlens array.

[0065] FIG. 5 is a cross-sectional view of an image sensor containing a
microlens array according to an exemplary embodiment of the present
invention.

[0066] Referring to FIG. 5, the image sensor containing the microlens
array according to the exemplary embodiment of the present invention may
include a substrate 100, a plurality of microlenses L formed on one side
of the substrate 100, and a plurality of photo-detectors P formed on the
other side of the substrate 100.

[0067] Here, the substrate 100 may be a semiconductor substrate formed of,
for example, GaAs, GaP, InP, InGaAs, sapphire (Al2O3), or GaN,
but the present invention is not limited thereto.

[0068] The plurality of microlenses L may be formed on one side of the
substrate 100 in positions corresponding to the plurality of
photo-detectors P, respectively, thereby forming, for example, a
predetermined microlens array.

[0069] Each of the microlenses L may be formed of a semiconductor material
layer stacked by gradually increasing an oxidation rate as the
corresponding microlens L becomes farther from the substrate 100. Each of
the microlenses L may be formed by selectively oxidizing the
semiconductor material layer.

[0070] For instance, the oxidation rate of the semiconductor material
layer may depend on the Al composition ratio thereof. The formation of
the microlens L may include gradually increasing the Al composition ratio
of the semiconductor material layer as the microlens L becomes farther
from the substrate 100 to increase the oxidation rate according to the Al
composition ratio as the microlens L becomes farther from the substrate
100. Thus, after completion of oxidation, an interface between an
oxidized region and an unoxidized region may form a lens shape.

[0071] In this case, a method of forming the semiconductor material layer
constituting the microlens L may be performed using, for example, an
analog alloy process or a digital alloy process, but the present
invention is not limited thereto. The analog alloy process and the
digital alloy process were described above in detail with reference to
FIGS. 2 through 4.

[0072] Furthermore, the microlens L may be formed as a spheric lens or
aspheric lens by appropriately controlling the oxidation rate (or Al
composition ratio) of the semiconductor material layer. A central portion
of each of the microlenses L may have a height of, for example, about 1
to 2 μm.

[0073] Meanwhile, the photo-detector P may be any apparatus, for example,
a photodiode (PD), which may externally receive light and convert the
received light into an electric signal.

[0074] For example, the photo-detector P applied to one embodiment of the
present invention may include a lower ohmic contact layer 400, an active
layer 500, an upper ohmic contact layer 600, a passivation layer 700, and
upper and lower electrodes E and E', which are formed on a substrate 100,
but the present invention is not limited thereto.

[0075] In general, the lower ohmic contact layer 400 may include, for
example, an n-type doped semiconductor layer, while the upper ohmic
contact layer 600 may include, for example, a p-type doped semiconductor
layer, but the present invention is not limited thereto.

[0076] The passivation layer 700 may be formed using, for example,
SiNx, SiO2, or a polymer material.

[0077] The upper and lower electrodes E and E' may include, for example, a
metal material, such as nickel (Ni), gold (Au), germanium (Ge), platinum
(Pt), or titanium (Ti).

[0078] In an image sensor containing a microlens array according to an
exemplary embodiment of the present invention, a predetermined microlens
array including a plurality of microlenses L may be formed on a substrate
100. Thus, light incident to each pixel of each photo-detector P of the
image sensor may be focused on a small photodetection region of the pixel
to increase the amount of light incident to the photo-detector P. As a
result, the SNR and resolution of the image sensor may be increased, and
the sensitivity of the image sensor may be improved.

[0079] Furthermore, a lens, which may be easily monolithically integrated
with an image sensor and have a high NA, may be manufactured, and a
subminiature or high-density microlens array may be manufactured, thereby
lowering costs for manufacturing high-performance image sensors and
improving the performance thereof.

[0080] FIG. 6 is a cross-sectional view of a microbolometer IR image
sensor containing a microlens array according to an exemplary embodiment
of the present invention.

[0081] Referring to FIG. 6, as compared with FIG. 5 that shows the
microlens array formed on one side of the substrate of the image sensor,
an image sensor and a microlens array may be monolithically integrated.
The image sensor of FIG. 6 is limited to a case where light is incident
toward a substrate.

[0082] Meanwhile, because a readout integrated circuit (ROIC) is formed on
a substrate of an image sensor or for other reasons, a microbolometer IR
image sensor 700 in which light is not incident through a substrate but
incident from above a detector (i.e., opposite the substrate) may be
typically provided.

[0083] In this case, the microbolometer IR image sensor 700 may typically
include thermal detectors instead of photo-detectors. Thus, the
microbolometer IR image sensor 700 may include an array of IR image
sensors (i.e., thermal detectors T), and each of the thermal detectors T
may include an electrode A, a reflection plate B, and an IR absorption
layer C disposed on a substrate 710. In this case, since light is
incident not toward the substrate 710 but toward the microbolometer IR
image sensor 700, a microlens array 800 according to an exemplary
embodiment of the present invention may be formed over the microbolometer
IR image sensor 700. In this case, it may be impossible to monolithically
integrate the image sensor and the microlens array. Accordingly, as shown
in FIG. 6, the microlens array 800 and the microbolometer IR image sensor
800 should be separately manufactured and hybrid-integrated.

[0084] Specifically, as shown in FIG. 6, the microlens array 800 and the
microbolometer IR image sensor 700 may be separately manufactured such
that the IR absorption layer C of each of the thermal detectors T is
spaced a predetermined distance apart from the corresponding one of the
microlenses L of the microlens array 800. Thereafter, the microlens array
800 and the microbolometer IR image sensor 700 may be hybrid-integrated
with each other using, for example, a flip-chip bonding apparatus.

[0085] Meanwhile, the structure of the microbolometer IR image sensor 700
applied to one embodiment of the present invention may typically include
an array of thermal detectors T, each thermal detector T including the
electrode A, the reflection plate B, and the IR absorption layer C formed
on the substrate 710. However, the present invention is not limited
thereto, and the structure of the microbolometer IR image sensor 700 may
have various other shapes.

[0086] The microbolometer IR image sensor including the microlens array
having the above-described construction may focus external light (i.e.,
IR light) incident to each of the microlenses L of the microlens array
800 and emit the light to a region of the IR absorption layer C of each
of the thermal detectors C to increase the amount of light incident to
the thermal detectors T. Thus, the SNR and resolution of the IR image
sensor may be increased, and the sensitivity thereof may be improved.

[0087] FIGS. 7 through 14 are cross-sectional views illustrating a method
of manufacturing an image sensor including a microlens array according to
an exemplary embodiment of the present invention.

[0088] Referring to FIGS. 1 and 7, a microlens-forming layer 200 may be
formed on one side of a prepared substrate 100 by stacking a
semiconductor material layer having a gradually increased oxidation rate.
A lower ohmic contact layer 400, an active layer 500, and an upper ohmic
contact layer 600 may be sequentially stacked on the other side of the
substrate 100, thereby forming a photo-detector forming layer 300 for
forming an image sensor (i.e., a photo-detector P).

[0089] In this case, a method of forming the photo-detector forming layer
300 is not limited to the above description. The photo-detector forming
layer 300 may be formed using any one of various other methods when the
used method is applicable to a method of manufacturing an image sensor
including a microlens array according to an exemplary embodiment of the
present invention.

[0090] Here, the substrate 100 may be a semiconductor substrate formed of,
for example, GaAs, GaP, InP, InGaAs, sapphire, or GaN, but the present
invention is not limited thereto.

[0091] Meanwhile, the photo-detector P may be any apparatus, for example,
a PD, which may externally receive light and convert the received light
into an electric signal.

[0092] The microlens-forming layer 200 may include a semiconductor
material layer stacked by gradually increasing an oxidation rate as the
semiconductor material layer becomes farther from the substrate 100.

[0093] For example, the oxidation rate of the semiconductor material layer
may depend on the Al composition ratio thereof. The microlens-forming
layer 200 may be formed by gradually increasing the Al composition ratio
of the semiconductor material layer as the semiconductor material layer
becomes farther from the substrate 100.

[0094] In this case, a method of forming the microlens-forming layer 200
may be performed using, for example, an analog alloy process or a digital
alloy process, but the present invention is not limited thereto. The
analog alloy process and the digital alloy process were described above
in detail with reference to FIGS. 2 through 4.

[0095] Meanwhile, an oxidation barrier layer 250 may be further formed on
a top surface of the microlens-forming layer 200 as needed. The oxidation
barrier layer 250 may inhibit vertical oxidation of an upper portion of
the microlens-forming layer 200 and allow only lateral oxidation of a
lateral portion of the microlens-forming layer 200 during subsequent
oxidation of the microlens-forming layer 200, thus resulting in formation
of a lens shape.

[0096] Referring to FIGS. 1 and 8, a predetermined region of the
microlens-forming layer 200 may be etched until the substrate 100 is
exposed, thereby forming a plurality of mesa structures M required for
forming microlenses L. The mesa structures M may be spaced a
predetermined distance apart from one another and have exposed lateral
surfaces. That is, the microlens-forming layers 200 having the mesa
structures M may be formed.

[0097] Here, the predetermined region of the microlens-forming layer 200
may be etched using, for example, a photolithography process, but the
present invention is not limited thereto.

[0098] For example, a method of etching the predetermined region of the
microlens-forming layer 200 using a photolithography process will now be
described in detail. To begin with, a photoresist layer may be coated on
the microlens-forming layer 200.

[0099] Subsequently, the photoresist layer may be exposed and developed,
thereby forming a photoresist pattern with a predetermined shape in a
partial region of the microlens-forming layer 200 to be etched, for
example, a region of the microlens-forming layer 200 corresponding to
each of regions where the plurality of photo-detectors P will be formed.

[0100] In this case, the photoresist pattern may be formed in one of
various shapes, for example, a circular shape or a polygonal shape such
as a square shape, according to a desired shape of the microlens L.

[0101] Thereafter, the exposed microlens-forming layer 200 may be etched
using the photoresist pattern as an etch mask, thereby forming a
plurality of mesa structures M on the substrate 100.

[0102] In this case, the exposed microlens-forming layer 200 may be etched
to expose a lateral surface of each of the mesa structures M until a
predetermined region of the substrate is exposed.

[0103] Referring to FIGS. 9 and 10, a lateral surface of each of the mesa
structures M may be oxidized, thereby forming a microlens L having a
radius of curvature in the center of each of the mesa structures M (i.e.,
each of the microlens-forming layers 200 having the mesa structures M).

[0104] In this case, the oxidation process may be a wet oxidation process
performed at a high temperature. Thus, each of the mesa structures M may
be oxidized in a lateral direction, thereby enabling the formation of the
microlens L without an additional process.

[0105] That is, since the oxidation rate of the microlens-forming layer
200 becomes higher as the microlens-forming layer 200 becomes farther
from the substrate 100, a rate at which the microlens-forming layer 200
is oxidized may be gradually increased, thereby forming the microlens L
at an interface between an oxidized region and an unoxidized region.

[0106] Afterwards, as shown in FIG. 10, a process of removing an oxidized
semiconductor material from regions other than the microlens L may be
further performed on the microlens-forming layer 200 having the microlens
L. In this case, for example, when the oxidation barrier layer 250 is
formed on the microlens-forming layer 200, the oxidation barrier layer
250 may also be removed. Meanwhile, the wet oxidation process may be
performed at a temperature of about 300 to about 500° C. for about
30 to about 200 minutes.

[0107] Referring to FIGS. 11 through 14, a plurality of photo-detectors P
may be formed on the other side of a substrate 100 having a plurality of
microlenses L to respectively correspond to the microlenses L.

[0108] The photo-detector P may be formed on the substrate 100 using
various known methods.

[0109] For example, as shown in FIG. 11, a predetermined region of a
photo-detector forming layer 300 formed on a substrate 100 may be etched
until a lower ohmic contact layer 400 is exposed, thereby forming a
plurality of mesa structures M' a predetermined distance apart from one
another.

[0110] Here, the predetermined region of the photo-detector forming layer
300 may be etched using, for example, a photolithography process, but the
present invention is not limited thereto.

[0111] Thereafter, as shown in FIG. 12, the lower ohmic contact layer 400
may be etched until the substrate 100 is exposed, thereby patterning each
region required for forming the photo-detector P.

[0112] The lower ohmic contact layer 400 formed between two adjacent ones
of the plurality of mesa structures M' may be partially etched, thereby
patterning regions for a plurality of photo-detectors P corresponding
respectively to a plurality of microlenses L.

[0113] In this case, the lower ohmic contact layer 400 may be etched
using, for example, a photolithography process, but the present invention
is not limited thereto.

[0114] Subsequently, as shown in FIGS. 13 and 14, a passivation layer 700
may be coated on the entire surface of the other side of the substrate
including respective regions for forming the plurality of photo-detectors
P, and upper and lower electrodes E and E' may be formed in each of the
regions for forming the plurality of photo-detectors P.

[0115] In this case, the upper and lower electrodes E and E' may be formed
by patterning predetermined regions of the passivation layer 700 formed
on each of the regions for forming the plurality of photo-detectors P to
expose each of the upper and lower ohmic contact layers 500 and 400.
Thus, the upper electrode E may be formed on each of exposed regions of
the upper ohmic contact layer 500, while the lower electrode E' may be
formed on each of exposed regions of the lower ohmic contact layer 400.

[0116] The plurality of photo-detectors P may be formed on the substrate
100 using the above-described serial process, but the present invention
is not limited thereto. The photo-detectors P may be formed using any one
of various other methods as long as the method used is applicable to a
method of manufacturing an image sensor including a microlens array
according to an embodiment of the present invention.

[0117] FIGS. 15 and 16 are optical microscopic images of a microlens array
manufactured using a method according to an exemplary embodiment of the
present invention.

[0118] FIG. 15 is an image of a circular microlens array, and FIG. 16 is
an image of a square microlens array.

[0119] For example, a ratio of the area of a microlens to that of a unit
cell of a microlens array may be referred to as a fill factor. In this
case, the circular microlens array of FIG. 15 has a fill factor of about
64%, while the square microlens array of FIG. 16 has a fill factor of
about 87%.

[0120] Accordingly, the square microlens array having a higher fill factor
may be adopted to maximize the efficiency of a microlens integrated with
the image sensor.

[0121] According to an image sensor including a microlens array and a
method of manufacturing the same of the present invention as described
above, a microlens can be manufactured using a semiconductor material,
thereby enabling manufacture of a lens, which may be easily
monolithically integrated in an image sensor and have a high NA, and
manufacture of a subminiature, high-density, polygonal microlens array.

[0122] Furthermore, according to the present invention, a microlens array
can be formed in an image sensor so that costs for manufacturing image
sensors can be reduced, the SNR and resolution of the image sensors can
be increased, and the sensitivity thereof can be improved.

[0123] It will be apparent to those skilled in the art that various
modifications can be made to the above-described exemplary embodiments of
the present invention without departing from the spirit or scope of the
invention. Thus, it is intended that the present invention covers all
such modifications provided they come within the scope of the appended
claims and their equivalents.